Electric circuit regulator



17,.1943- T. P. KINN El AL ELECTRIC CIRCUIT REGULATOR Filed March 17 INVENTORS Theodore K0727 er/7d Dar/a R Task fa)?! wnmzssss;

ATTORNEY Patented Feb. 17, i94

UNITED STATES; PATENT omcs v ELECTRIC omom'r REGULATOR Theodore P. Klnn and David R. Tashjian, Baltimore, Md., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application March 17, 1945, Serial No. 583,292

3 Claims.

Our invention relates to arrangements for tuning resonant circuits, such. for example, as those of oscillation generators, and'in particular relates to electrically operated arrangements for adjusting and maintaining the frequency of such circuits at a predetermined desired value. An arrangement for a similar purpose is shown in our application for System for remote control of an oscillator, Serial No. 583.293. filed March 17. 1945.

For many purposes, particularly in the radio art, it is desirable to maintain the frequency of circuits such, for examplaas the output of an oscillation generator, at a constant value throughout an extended period of time notwith-,

ances which determine the frequency of resonance of the circuit are of large size, and, while arrangements are known for stabilizing the frequency of resonant circuits against their tend ency to vary, such arrangements become extremely expensive and cumbersome when they are ap-' plied directly to the large size capacitors and inductances involved in high power circuits. Likewise, it is usually desirable to employ large amounts of power for output-apparatus such as antennas which are likely to be located in places where they are necessarily capacitors and inductances of the resonant circuits have to be manually adjusted by the supervising operator, it is necessary to transmit the power circulating through these capacitors and inductances through relatively longtransmission lines to reach the antennas and other output apparatus just mentioned.

In accordance with our invention. we have devised arrangements by which the settings of the large-size capacitors and inductances may be remotely controlled from resonant. circuits of small power. The arrangements are such that the electrical apparatus'automatically adjusts the dis tant large-size capacitors or inductances whenever the frequency of the current in the latter deviates for any reason from the resonant frequency to which the small-sized, control-resoe nators are adjusted. Because of the small size of It is also desirable to be able to adjust at stabilize their frequency by methods well known in the radio art. Also, in the generation of large amounts of power, it is customary to employ several stages of amplification, each of which has to be separately tuned. By means of the arrangements of this invention the intermediate stages of amplification are eliminated permitting tuning control of a high power generating circuit with a minimum number of controls,

Since the'small-size control resonators may be separated by electrical lines of relatively small capacity from the distant large-size tuning elements which they control, it is possible to locate the latter and all the equipment such as highpower generators involved in the high-power circuits close to the output elements such as antennas retaining only the small-sized control elements at the stations to be occupied by the supervising operators. This possibility presents a go tremendous advantage in certain fields of radio operation.

One object of our invention is accordingly to provide arrangementsfor adjusting tuning elements of electric circuits by electrical control to maintain resonance with control-resonators.

Another objectof our invention is to provide an arrangement for adjusting tuning elements of an electric circuit by a remote control system so that they maintain resonant adjustment which so coincides with that or, controlresonators positioned at a substantial distance from them.

Another object of our invention is to provide an arrangement by which tuning elements of an electric circuit may be moved to follow changes distant fr m the of adjustment in control resonators to which they supervising operators of the equipment. If the are linked only by electrical control circuits.

Still another object of our invention is to increase the efiiciency of operation, or decrease the initial expense, of radio installations of substantial power by making it possible to locate the high-power elements of'the installation at a distance from the supervising operator.

Other objects of our invention will become apparent upon reading the following description 5 taken in connection with the drawings, in which:

Figure 1 is a schematicdiagram of a control circuit which operatesfto maintain the frequency of an oscillation generator in resonance with the setting of a resonantcontrol circuit whatever may be adjustment of the'latter, and

Fig. 2 is a schematic diagram of an alternative for a part of the circuit or Fig.1 which controls a search relay therein.

Referring in detail t0 Fi 1, reference characthe control-resonators theyshow little tendency 38 $81 I is a standard oscillation generator which to vary in resonant frequency, andinsofar as-they do. is comparatively easygand inexpensive to may be of any type employing a resonant conrol circuit, but which forpurposes ofillustration circuit may be inductively coupled to "inductor 4. The anode of the tube l isconnected through a capacitor B to one termi iialof the inductor i and is likewise connected inr iig r en i di ete e l to the positive terminal 3-]- of a source of direct-current voltage having its negative terminal grounded. The control electrode of the'tulo'el is connected through a resistor 8 to itscathode and through a capacitor 9 to the otherterminal of the inductor 4, Theinductor 4 and capacitor 5 constitutejthe fre'aq'uentry-setting; tank cir' cuit for the oscillatbr I; and the capacitor 5 is connected by a suitable mechanism tothe shaft of a' motor having] an armatiire H and a field WindinglZ which,is relay-operated to maintain the desired adjustment for resonance "of the tank circuit 4, 5. Loosely coupled to the inductor 4 is a secondary winding l3 having terminalsA, B which are adaptedjior connection, for example through a'low ii'npledai ce transmission line', to the terminals Al, Bl ofa' primary'winding l4 located at any desireddisftance fromthe' oscillator ,L A work circuit l5'which may, ,for example, lead to a radiating antenna ijs lilgewisepoupled to the inductor 4. Loosely coupled With the primary winding is, ,aseqondary winding 16 shunted by a variable capacitor, ll and having one terininal connected through a capacitor 13 to a common terminal or apaino'i substantially equal windings l9, 2!; each'of which'i's loosely coupled to thepwinding [Bl Theroutside terminals of'the windings, l9, 2,lare shuntedbya variable capacitor 22 which is preferably connected tovmove in unison with thefcapacitor' l1 ahd to resonate the windings l9, 2 'I, to thisanie freq iency s thefca pacitor l1 resonateswiththe winding 3. "A pair of rectifiers 23; are likewise'connected between the irespectivef outside ,terminals of the windings l9; 2! and a resi'stqr 25,;which has its mid-point connected tc the coin p v d V tially equal pairoipapacitorS 21 28, whi ch"shunt the resistor25'. ,Onetenninal of the resistor 25 is connected to ground while the other terminal is connected througha resistor 25th the input circuit of aconiientional aiternatin'g' eurie nrem plifier 33. Those 'skille'din theartwill recognize the network l3 through 28 as "a. Seeley"fclis'criminator in which the'voltaabetween the outside terminals of the tor 25 willbe zerowhen the on'termiilal' oiia'jsubstanfrequency of. the, current flowihginrthe winding It is that to whijcritheii ctance wand 20, and capacitor narereseii'en "and in which a voltage of one polarit'y'jwill exist across the .outside terminalsof the resistor 25 whentheffree quency of the chrreritlih! the winding ldis'less than the resonant neqiieii'cyi oi thegelements it, ii, and is ofth'e. opp ite polarity when the frequency of the current in, theiwlnding I4 is greater than the resonant' freduencyflo'f theelemen'ts l9 to 22. While we;, i avechosejn t'e; illustrate our invention by employi'rig 'fa" discriminator of the Seeley type, it will be evident ftofthose skilled in the art that thisismerely representative of dis criminators generally; which have the property 7 of producing adirect clirrent voltage across their output terminals which 1 ls of Tone polarity when their input ffediificil is lss tiliarl that for which trier property known electrode of an electron tube 33 which is what is known in the art as a "reactance tube; that u is to sayritis' a tube which draws a current e. voltage across its terminals, like a reactance in an alter- This tube has the furati the amount of current flowingthrough it increases when the direct-current voltage iinpress'ed'on its control electrode increases, and vice versa; in other words, it func tions as a reactance, the magnitude of which varies with the direct-current voltage impressed on its control electrode by the amplifier 30 through rectifier3l; v v W I While we have described the amplifier 30 as a conventional alternati ng -current amplifier; it will bepossible to substitute for it aconventional direct-current amplifier; but'we prefer to employ an alternating current amplifier becauseof the difficulty such as poors'tabilization which charf acterizes present direct-current amplifiers. In order that the amplifier 30 shall operate as an alternating urrent device, we iinpress on its input circuit, in addition to the direct-current output of the discriminator Iii- 28 aninput from analternating current source such as an oscillation generator 34 having a frequencysuitable for efficient operation of the amplifier 30. However, it will be obvious to those skilled in the art that th e alternating currnt input to be im pressed on the amplifier 30may be derived from any convenient 'source of alternating voltage through suitable blocking-condensers; such an alternative voitage source may; for-example, be some alternating circuit of the network being frequency controlled; Thetube 33 is connected to 'actas a reactance'tube by connecting a capacitor 35 b'etween its control electrode and cathode and'aresistor- 38' in series with a'b l'ocking condenser '31 between its control electrode and its anode HoweverQother reactance tubes are d we do not wishto limitour inven tion' to the one herein described in detail. p

r The anode of thetube-33 is connected through a capacitor 33 to the contrbl electrode or the oscillation 7 generator I V As a result, the reactance tube 33 is, in effect, connectedto addits reactance to the tank circuits 4', Sandtd vary the resomatingfrequency of the generator l'up or down in correspondence with the changes in the cur-- rent between the anode' 'and cathode of tube 33.

A moments' consideration willshow that when the frequency at which thetube 'l is'operating corresponds with the resonant frequency of the e s 1 to lie ihes sqri inato t e voltageof that discrimination will impress zero direct-current volt'ag on the 'arn'plifier 3D, The

Variation with load off the winding 15 the fref u icyiofrjtlie s a de' 'e' e c r nt in the winding ft'w l no l'onger correspond in ire lle cy chanc theindu'ctances "l9 and 2 l ,andicapacltor-H'ofthediscrimlnator, and

a direct 'cui'rent voltage will be impressed by the output terminals of the discriminator of the amplifier 39. This will change the value of the di+ rest-current voltage impressed on the amplifier and hence change the unidirectional voltage acting on the reactance tube 33 and vary the value of its plate current to change the eifective reactance which it impresses on the resonant circuits 4, 5. By proper choice of polarity of the discriminator I6-28 the alteration of the equivalent reactance of the tube 33 may be made such as to increase the resonant frequency of the tank circuits 4, 5. This increased frequency will continue to produce a voltage on the output of the discriminator Iii-28 and to alter the plate-to-cathode current-of reactance tube 33 until the resonant frequency of the tank circuit of oscillator I comes back to coincidence with the resonant frequency of the discriminator circuit I9, 2|, 22.

Not only will the alternating current drawn by the reactance tube 33 vary when the directcurrent voltage impressed from amplifier 30 on its control electrode changes, but also the amount of direct current which its plate circuit draws from the direct-current supply will likewise change. The connection from the plate of the tube 33 to the direct-current supply is through an inductor 4| and through one winding 42 Of a balanced relay 43, the other winding 44 0f which is fed from some direct-current source such as B through a resistor 45, The windings 42 and 44 are so proportioned that when the discriminator I628 impresses zero voltage on amplifier 30, the windings 42 and 44 exert equal and opposite pulls on a movable contact 46 of the relay 43. When the net force on the movable contact 46 is zero,-

it stands inan intermediate position between a pair of co-operating contacts 41 and 48 of the relay 43. The contact 41 is connected to the positive terminal BI of the direct-current source and the contact 48 is connected through ground to the negative terminal of that source. The con-' tact 41 and the contact 48 are respectively connected to the opposite terminals of a pair of serially connected relay windings 49 and 5|. The winding 49 is positioned to actuate a pair oi. con-' tacts 52 from open to closed position, and a pair of contacts 53 from closed to open position, when winding 49 is energized. The winding 5i is similarly positioned so that when energized it moves a pair of contacts 54 from open to closed position and a pair of contacts 55 from closed to open po sition. The contact 41 is likewise connected to one terminal 56 of a limit switch 51, the other terminal 58 of which is connected through ground to the contact 48. Homologous contacts of contact-pairs 52, 53 are connected together to one brush and homologous contacts of pairs 54, 55 are connected together to the other brush of the motor armature -I I, The remaining terminals of the contacts 53 and 55 are connected together to one side of the field winding l2 which is energized from a direct-current source 59.

Between the mid-terminal of the winding 2I and the mid-terminal of the resistor 25 of the discriminator is connected a winding 6| of what may be termed a search relay which moves two pairs of normally open contacts 62, 63 from open to closed position, and moves two pairs of normally closed contacts 64, 65 to open position when a sumciently large current flows through it.

. Homologous contacts of the contact pairs 62, 64

are connected to the common terminal of the windings 49, 5I and homologous contacts ofthe pairs 69, 65 are connected to the remaining concontacts 64 and 65.-

6 ta'ctsot the' pairs 52, 54 previously mentioned.- The remaining contactfrom the pair 62 is connected-to the contact 46, the remaining contact from the pair 63 is connected to the mid-point of the voltage source 59; the remaining contact from the pair 64 is connected to the movable blade of the limit switch 51, and the remaining contact of the pair is connected to the opposite end of voltage source 59 from contact 55. It will be noted that .withthe'above-described connections the-brushes of armature II are short-cincuited when windings 49 and 5| are both deenergized,- and also when they are both energized.

The limit switch 51 is mechanically attached to the movable element of the capacitor 5 so that it is pushed into contactwith the contact 56 when the capacitor 5 is moved toone of its extreme positions, and thereafter remains in contact with56 until the capacitor 5' is moved to its other ex-' treme position, at-which time the movable blade of 51 is moved out of contact with 56 and into contact with 58.-

The mode ofoperation'of the foregoing relaysy'stein will-now be described. We will consider first the conditions existing when the frequency of the oscillator I is notfar different from that to which'the elements I9 to 22 of the discriminator aretuned. Under'such conditions a substan tial current will flow through the winding 6I and will close thecontacts 62 and 63 and open the With the contacts 64 thus open, the movable member of the limit switch 51 is disconnected'-from circuit and the positionof thatlimit' switch plays no part in the operation of the relay system. Onthe other hand, closure of 'th'e contacts 63 will connect the upper contacts of therelays Hand 54 to the mid-point of the voltage source 59. However, this will not. in itself. energize the motor II, I2 because, until one ofthejwindings 49 or 5| is energized and the other deenergized the brushes of the armature II are short-circuited through either the contacts 53,55 or-the-contacts 52, 54.

. However, closure of the contact 62 as just de scribed connects bothwindings 49 and 5| to the movable contact 46 of relay 43. As long as the movable'contact 46 of relay 43 remains in its midposition," it will impress no voltage on winding 49- or 5|, butcurrent will flow in series from the' positive terminal B+ of the voltage source through windings 49, 5I-to ground and thence into the negative terminal of -thevoltage source. This energization of the windings 49, 5| will close thecontact pairs .52, 54 and open the contact pairs 53, 55; but this still leaves the brushes 0f the armature --I I short-'circuited through contact pairs 52, 54'and hence the armature? I I will not moveto alterthe setting of capacitor 5. The foregoing is the" condition of the apparatus when the frequencyofthe oscillator I coincides with the resonant frequency of the discriminator ele-' ments I9 to 22. i I

Ifthe frequency of the generator I is altered slightly from the resonant frequency of the ele-- ments I9; 2| 22, the currentlflowing throughwindi'ng 42 ofrelay 43 alters and the latter no longer exerts a force exactly equal to that of the winding 44 on the movable contact 46.. If the current drawn by reactance tube 33 increases as a result ofthediscrepancies between the frequencyof oscillator I and the resonant frequency ofzthe elements I9, 2|, 22, the movable contact 46 will be pulled downward into incidence upon contact-48. i-As'a result of this the winding 5i will be *short-circuited .throughcontacts 46. 46

andt'BZrand the'contact lipaii- 54 i willzopen while contact pair 55 closes; 'The.=lowery-brushz of the armature II will thus be connectedfito-one end terminal of'the voltage source59 through contacts. 55 while the upperibrushof; armature: I I will remain connected through contactrpair, -52 and-closed contact pair to the mid-pointcf the voltage source 59. Half the voltage of. the source 59 will thus be impressed on-the armature II thereby causingitto slowlyfirotate in-one direction. By properly poling the armature-I1 and field winding I2; this --d-irection--of rotation can be made such-as to move the capacitor -5i-n the direction to bring the frequency (if-oscillator I back into coincidence withthe resonant frequency of elements I 9, 2t, 22 citric-discriminator.-

If, in contrast to the -foregoing,,gthescurrent drawn by reactance tube 33 through winding asa-result of the discrepancy between theft-requency of oscillator I-andthe elements t9, 2|, 22 had decreased, the strengthbf winding would have overcome the force exerted by-winding; 42 and movable contact 46 would move intoincidence with contact 41 of relay 43. Hadthishappened, thewinding 49 would have-been shortcircuited through contacts 46,.4l-and 82 while winding -5I remained energized, and the contact pair 52- would "have opened when contact pair 53. reclosed. The lower brush of motor. would then beconnected throughcontact pairs 54 and B3to the mid-point of voltage source 59 while the-upper brush of armature I-I would-be connected through contact air 5 3-to the.-r-ighthand terminal. of voltagesourceSB. Thebrushes of thearmature II would accordingly be connected with the'opposite polarity to thatoccurring when contact 45 movedinto incidence with contact 48 asabove described, and the-capacitor 5 would bemoved in the sense which would tend to bring the frequency of oscillator I. back-into coincidence; with the resonant frequency of elements t9, 2-I, 22. r

It will thus be evident that the relaysystem above described acts to; correct, the frequency of-the oscillator I by the action 'ofreactanCe-tube 33' so long as winding BI is traversed-by suffici'ent current to close the contact pairs Hand 63, However, the amount of iequivalent reactance which the tube 33 can reflect intothe'tankcircuit 4, 5 is limited by certainfactorsgso that the arrangement just described cannotzbe relied upon to furnish the necessary correction if the "frequency of oscillator I departs too far from the resonant frequency of elements I9, 24,22 'of'the discriminator. When such a widejdeparture occurs, however, the current throughthe winding SI will decrease and the contactpairsfl; 63,164, 65 will then fall to the condition shown in Fig. l. The relay 43 then temporarily loses control of the motors I I, I2 and the limit switch ,5lsacquire's such control. 7 l

It will be seen that, with the contact'pair '64 closed, the common terminal of the windings, 5|; is connected to the movable bladerof the limit switch 51-. As. previously; stated, this movable blade is in contact either through contact 56' with the positive terminal of voltage source vB or through contact 58 with the ground. Ifthe movable blade is in incidencewithcontact 55', the winding G5 is short-circuited: through contact pairs 64 and 55 while winding 51 is 'energizedifrom the positive terminal of voltage source BS through ground. As a result, contact pairs.ll2, 53' are in the positionshown in the drawing, .whilecontact air' 54 is closed andlcontact pairr55. open.

- 8 The lower brush or the armature II is therefore lnrcontact through contact pairs 54 and with the left-hand terminal of the voltage source 59 when the upper brush of armature II is in corn tact throughcontact pair 53 with the right-hand terminal of voltage source 59. Thus thefull voltage of thesource 59 is impressed on the ar maturev II to rotate it with comparative rapidity andthereby alter the setting of capacitor 5 in the tank circuit of oscillator I. If the direction of rotation of armature II is such as tomove the-v frequency of the oscillator I toward coinci deuce with the resonant frequency of the ele: ments 15; I! of the discriminator, the frequency of oscillator I will soon become close enough to such resonant frequency so that substantial current will begin flowing through winding GI and return the 'relaysystem to the condition firstdescribed in which the relay 43 governs the motor II, 12in such a way as to slowly vary the frequency, of oscillator I into coincidence with the resonance ofelements I9, 2|, 22. r i On the other hand, if the direction of rotation of the armature i I is such as to move thecapacitor 5,-to increase the discrepancy of frequency betweenoscillator I and the resonant frequency of the discriminator elements I9,'2 I 22, the winding; BI will remain temporarily deenergized.-

However, the capacitor I will soon reach one of its extreme positions, and thereby move the limit switch 51 to contact with the other terminal 58. When this happens the winding 5I will be shortcircuited and the winding 49 will be energized, thereby causing closure of the contact pair 54,- opening of the contact pair 55, closure of the contact pair 52, and opening of the contact pair 53; This will reverse the polarity with which the brushes of armature II are connected .to voltage source 59; the motor will begin to rotate in the opposite direction and thereby move thecapaci-tor 5 to alter the frequency of oscillator I in the opposite 'sens elto which it was being .varied before the capacitor 5 reached its end position and thereby changed the position of limit switch 51-, I

Varying the frequency of oscillator l in this opposite sense will necessarily soon bring it into close coincidence with that of the resonantfrequency of discriminator elements 15, 2|, 22, at

which time the search relay windings BI will be reenergized to disconnect the winding 49, 5| from control of the limit switch 51 and put them under control of the relay 43. It has already been shown that once the motor II, I2 is under controlo f relay 33 it moves the capacitor 5 in the right amount and direction to'bring the oscilla-tor I into consonance with the resonant fre-; quency of discriminator elements I9, 2|, 22.- It is thus evident that the relay system above described is capable of adjusting thefrequency of thepsci'llator -I toconsonance with the resonant frequency of the discriminator elements I9, '21-, 22"re'gardless of how much the frequency of the oscillator I may accidentally depart "from the said consonance; While we have described this adjustment as carried out by mechanical adjust? ment of capacitor 5, this is only illustrative of any method of tuning known to those skilled in the art. I 1'- It will be further evident that the operating frequency of the oscillator I may be changed from a given value to any other desired value by manu ally adjusting the elements I5, I'I,q-I9, 21, 22 oi the discriminator so that they will be resonant at the new desired frequency. The relay system will then adjust the capacitor until the oscillator I operates at the new frequency.

It will further be obvious that the discriminator elements It through 28 may be located at a control point distant from the remaining elements of the above described system and that the only current channels required to operate the system as above described is the one connecting the discriminator with winding 6|, the one connecting windings I3 and I4, and the one connecting it with amplifier 30, Neither of these channels needs to carry anything except the small currents necessary to energize the winding of the relay in the first case, to energize the discriminator in the second, and to impress bias voltage on an amplifier in another case; hence they may comprise small conductors and cheap construction.

It may be noted that variation of capacitor 5 roughly adjusts the frequency of oscillator l, and fine adjustments are made by varying reactance tube 33. Where fine adjustments are not needed the elements 29 through 48 and 62 which employ the reactance tube 33 to finely adjust the frequency of oscillator I may in fact, be omitted. The relay control system for varying capacitor 5 is but an illustration of many that might effect its purpose.

It is also within the provisions of our invention to substitute, for the channels just mentioned as connecting the control point to the rest of the circuit, a radio link comprising a radio transmitter, located near the oscillator l and modulated by the currents being transmitted through said channels, and a receiver located at the control point and adapted to produce separate currents proportional to the respective currents in said channels. 7

There are a number of other points at which the relay 6| may be connected to the discriminator circuit than that just described. For example, the common terminal of the windings l9, 2| and the midpoint of resistor may be shortcircuited and the winding 6| connected across the capacitor 28, preferably with the interven-- tion of an amplifier. Figure 2 illustrates such a connection of winding 6|, the rest of the circuit and its connections to the elements shown in Fig. 2 remaining unaltered and so requiring no further description.

While we have described certain specific embodiments of the principles of our invention to comply with the patent statutes, there are numerous other ways of applying these principles specifically which will be evident to thoseskilled in the art.

We claim as our invention:

1. In combination with a resonating electric circuit having at least one component capable of being adjusted in value, a motor coupled to vary said element, a relay means arranged to electrically couple said motor for one sense of rotation in one position and for another sense of rotation in another position, means adapted to produce a voltage of one polarity when resonant current in said resonant circuit differs from the resonant frequency of a portion of the lastmentioned means in one sense and to produce voltage of an opposite polarity when the frequency of said resonant current differs from that of said portion in an opposite sense, means for causing the polarity of the last-mentioned voltage to govern the Sense of rotation of said motor when the difierence of said frequencies is below a predetermined magnitude; a limit switch 10 i I for said variable element and means for causing said limit switch to control the sense of rotation of said motor when the difference of said frequencies is above a predetermined magnitude.

2. In combination with an oscillation generator, a tank circuit therefor having an adjustable reactance, a motor for adjusting said reactance, a discriminator having an output voltage of one polarity when the frequency of said tank circuit departs from the resonant frequency of said discriminator in one sense and the voltage of the opposite polarity when the resonant frequency of said tank circuit departs from the resonant frequency of said discriminator in the opposite sense, means responsive to the output voltage of said discriminator for rotating said motor in a direction corresponding to the polarity of said output voltage when the departure of said tank circuit frequency from the resonance frequency of said discriminator is below a predetermined magnitude, and means for transferring the control of the direction of rotation of said motor to a limit switch operated by said variable reactance when said departure is greater than said predetermined magnitude.

3. In combination with a first oscillation generator, a tank circuit therefor having a variable reactance, a motor for varying said reactance, a second oscillation generator, a discriminator having a resonant circuit and having an output voltage which changes polarity when the frequency impressed on said discriminator of said tank circuit passes through the frequency of said resonant circuit, means for impressing the beat frequency of said first and second oscillation generators on said discriminator, a reactance tube controlled by said discriminator to compensate said variable reactance, a balanced relay adapted to cause energization of a second relay when the plate current of said reactance tube exceeds a predetermined value and to energize a third relay when the plate current of said reactance tube falls below said predetermined value, energization of said second relay causing rotation of said motor in one direction, means for causing energization of the third relay to rotate said motor in the opposite direction, a limit switch for said variable reactance connected through a fourth relay to cause rotation of said motor in one direction for one position of said limit switch and in the opposite direction for the other position of said limit switch only when said fourth relay is in one position, said limit switch being incapable of producing rotation of said motor when said fourth relay is in another position, and means for moving said fourth relay from said one position to said other position when the departure of said beat frequency from the frequency of said discriminator exceeds a predetermined value.

THEODORE P. KINN.

DAVID R. TASHJIAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

